Cigna wants to buy Express Scripts for $67 billion. Cigna is one of the country's largest health insurers, and Express Scripts is one of America's biggest pharmacy benefits managers, which administer drug plans for more than 266 million Americans with employer and government health insurance.

If the deal clears regulators, it would be the latest merger to disrupt the health care industry, which has been under heavy pressure in recent years as the federal government, employers and others attempt to control soaring health care costs. Drug prices, in particular, are a target, and pharmacy benefits managers are key players in that arena since they negotiate rebates from drug manufacturers and discounts from drugstores.

The merger could give Cigna more control over drug prices, which is a major issue for insurers. Rival insurer Anthem said last year that it would partner with CVS to start its own prescription drug plan manager. And UnitedHealth already has its own drug benefits manager, OptumRX.